Sahih al-Bukhari 3942
صحيح البخاري#3942SahihNarrated Abu Musa:When the Prophet (ﷺ) arrived at Medina, he noticed that some people among the Jews used to respect Ashura' (i.e. 10th of Muharram) and fast on it. The Prophet (ﷺ) then said, "We have more right to observe fast on this day." and ordered that fasting should be observed on it
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 63:166